Regulations for CO2 emissions have been considered in recent years to deal with atmospheric pollution and global warming. Particularly in the automobile industry, reduction in CO2 emissions is highly expected in association with the spread of hybrid electric vehicles and electric vehicles. As for motor driving power sources for these types of vehicles, development of high-performance secondary batteries is being carried out. In particular, since a large battery capacity and a high cycle property are required for such secondary batteries for driving motors, lithium ion secondary batteries having high theoretical energy are gaining increasing attention among other types of secondary batteries.
Lithium ion secondary batteries generally include: positive electrodes each containing a positive electrode active material applied to both surfaces of a positive electrode current collector by use of a binder, negative electrodes each containing a negative electrode active material applied to both surfaces of a negative electrode current collector by use of a binder, and electrolyte layers through which the positive electrodes and the negative electrodes are connected, wherein these elements are housed in battery cases.
The battery capacity and output performance of these types of lithium ion secondary batteries largely depend on the active materials selected for the positive electrodes and the negative electrodes. Among the active materials, solid solution materials in which Li2MnO2 and LiMO2 (wherein M represents a transition metal element such as Ni and Co) are present in a solid solution state, have attracted attention as the positive electrode active materials in recent years because of their high theoretical capacity, thermal stability and cycle property.
For example, Patent Literature 1 discloses a cathode composition for a lithium ion battery that is a solid solution type material including an Li-M1-Mn type composite oxide represented by a predetermined composition formula, and that is in the form of a single phase having an O3 crystal structure.